Daily rhythm of some blood parameters in two Amazonian fish, Astronotus ocellatus and Brycon amazonicus

We evaluated the effect of the light-dark (LD) cycle on energy expenditure indicators for two species of Amazonian fish of commercial interest, oscar (Astronotus ocellatus) and matrinxa (Brycon amazonicus). Fish were exposed to a 12:12 h LD cycle, with lights on at 'zeitgeber time' (ZT) = 0 h. Six animals were used for blood collection every 4 h for 24 h (six time points, three in the light period and three in the dark period). Most haematological parameters exhibited daily rhythms with the acrophase at the end of the dark phase or the beginning of the light period for both species, which may be related to the greater energy demand of these species during the day. For A. ocellatus, triglycerides and total protein showed acrophases at ZT = 13:32 +/- 2:45 h and ZT = 23:42 +/- 1:55 h, respectively, while the other plasma parameters showed no significant daily differences. In B. amazonicus, significant rhythms were observed for glucose levels (acrophase at ZT = 04:16 +/- 2:47 h), triglycerides (ZT = 6:51 +/- 4:06 h) and total cholesterol (ZT = 23:22 +/- 3:45 h). However, total protein and cortisol levels did not show rhythmicity in this species. Our results highlight the importance of the evaluation of biological rhythms for plasma physiological parameters related to energy expenditure in species with commercial interest and susceptible to stressful situations in farming conditions.